The effects of paper/substrate noise are significantly reduced in
multi-focal zone laser scanning systems by processing analog scan data
signals with a scan data signal processor having a plurality of pass-band
filters and amplifiers that are automatically selected for passing only
the spectral components of an analog scan data signal produced when a bar
code symbol is scanned at a particular focal zone in the laser scanning
system. Two or more different pass-band filter structures can be provided
for use in the scan data signal processor, wherein each pass-band filter
structure is tuned to the spectral band associated with a particular
focal zone in the laser scanning system. When a bar code symbol is
scanned by a laser beam focused within the first focal zone or scanning
range of the system, the pass-band filter structure associated with this
focal zone or scanning range is automatically switched into operation.
Only spectral components associated with the produced analog scan data
signal and noise existing over this pass-band are allowed within the
analog signal processor. By virtue of the present invention, first and
second derivative signals can be generated and processed to produce a
corresponding digital scan data signal for use in subsequent digitizing
and decoe processing operations, without compromising system performance
due to the destructive effects of thermal and substrate noise outside the
spectral pass-band of interest for the bar code symbol being scanned.